You can install soundfont files on your Mac and then use those sounds in Garageband or other music apps. Soundfonts are instruments created from real instrument samples.

1. Open Finder
2. Select the *Go* menu then *Go to Folder*
3. Type ~/Library/Audio/Sounds/Banks
4. Create a new folder, for example *Guitar*
5. Copy your SF2 file into the new folder

Now the soundfont should be available in any program that supports them, including Garageband.

See my other instructions on How to use a Soundfont in Garageband 10.